Dioxane adduct of 2-hydroxynaphthalene-6-carboxylic acid and process for the preparation thereof

ABSTRACT

The invention relates to the dioxane adduct of 2-hydroxynaphthalene-6-carboxylic acid which adduct comprises 1 mol of 1,4-dioxane and about 2 mol of 2-hydroxynaphthalene-6-carboxylic acid per mol. The invention further relates to a process for the preparation of this adduct, wherein the 2-hydroxynaphthalene-6-carboxylic acid is dissolved in dioxane or a mixture thereof with water or with an organic solvent and then allowed to crystallize.

DESCRIPTION

2-Hydroxynaphthalene-6-carboxylic acid is produced industrially by amodified Kolbe-Schmitt reaction. In this process the potassium salt of2-naphthol is reacted under pressure at 200°-300° C. with carbon dioxide(US-PS 4,329,494, EP-PS 53,824, US-PS 1,593,816, US-PS 4,287,357 andEP-PS 81,753). This reaction gives rise to considerable amounts ofdecomposition products (tars and resins) which are difficult to separateoff, as are the byproducts (2-hydroxynaphthalene-3-carboxylic acid and2-hydroxynaphthalene-3,6-dicarboxylic acid) also formed.

After separation of 2-naphthol, the crude acid has hitherto been workedup in a costly manner, viz. by extraction, adsorption on carbon andprecipitation from water.

It has now been found that 2-hydroxynaphthalene-6-carboxylic acid(abbreviated in the following to "2,6-acid") can be reacted with1,4-dioxane to form an adduct which comprises about 2 mol of 2,6-acidand 1 mol of dioxane per mol. The adduct crystallizes fromdioxane-containing solution in the form of well-formed column-like andplate-like crystals which can be separated from the mother liquor bysedimentation, filtration or centrifugation.

One subject of the invention is this adduct itself. A further subject isa process for the preparation of an adduct from2-hydroxynaphthalene-6-carboxylic acid and 1,4-dioxane, wherein2-hydroxynaphthalene-6-carboxylic acid is dissolved in 1,4-dioxane or amixture thereof with water or with an organic solvent and then allowedto crystallize.

Industrial 2,6-acid may be used directly for the preparation of theadduct.

The 2,6-acid is preferably dissolved in a mixture of 1,4-dioxane andwater having a 20-90% by weight dioxane content with heating and issubsequently allowed to crystallize out on cooling. The crystallineproduct is, for example, filtered off and dried at about 50° C. in vacuoin a stream of nitrogen.

Instead of a mixture of dioxane with water, a mixture of dioxane with anorganic solvent may also be used. It is even possible to use water andorganic solvents miscible with it at the same time as components of themixture; for example, the dioxane adduct may be allowed to crystallizeout from a ternary mixture of dioxane/water/lower alcohols ordioxane/water/polyethylene glycols. The dioxane content of the mixtureswith water and/or organic solvents is generally at least 10% by weight,preferably at least 20% by weight, in particular at least 30% by weight.

The adduct may be stored indefinitely at room temperature. If it isdesired to recover dioxane-free 2,6-acid therefrom, it is sufficient toheat the adduct to about 100° C. in vacuo. In this way the loosely bounddioxane is virtually completely split off, furnishing pure 2,6-acid asamorphous white powder or finely divided granules. Dioxane-free 2,6-acidmay also be recovered by recrystallization of the adduct from anothersolvent, for example dilute ethanol. Dioxane-free 2,6-acid mayfurthermore be obtained by dissolving the adduct in sodium hydroxidesolution, followed by precipitation with dilute mineral acid.

The special properties of the adduct according to the invention make ita valuable intermediate. Thus, 2-hydroxynaphthalene-6-carboxylic acidwhich could previously be isolated in pure form from the reactionproducts of the Kolbe-Schmitt reaction only with difficulty, can beprepared in a high degree of purity via the adduct. The possibility ofvirtually completely freezing 2,6-acid from impurities such as2-hydroxynaphthalene-3-carboxylic acid,2-hydroxynaphthalene-3,6-dicarboxylic acid and 2-naphthol which areformed in the synthesis as byproducts, via the adduct, represents asurprising step forward compared with the conventional troublesomeprocess for purification of carboxylic acids. Thus, for exampleaccording to US-PS 2,189,726, carboxylic acids may be purified bymultistage washing with water and heating or by steam distillation.

The adduct may be employed as coupling component for azo dyes, 1 mol ofadduct being reacted with 2 mol of a diazo component. The dioxane setfree in the reaction may be used to dilute and homogenize the reactionmixture.

The adduct is of special significance for the production of very pure2-hydroxynaphthalene-6-carboxylic acid ("fiber grade" quality), requiredfor polyesterification with organic hydroxycarboxylic acids such asp-hydroxybenzoic acid. The pure polyesters obtained in this manner arevaluable starting materials for the preparation of plastics or fibers(US-PS 4,393,191).

EXAMPLES

50 parts of crude 2-hydroxynaphthalene-6-carboxylic acid (obtained bythe Kolbe-Schmitt method) were dissolved in a mixture of 400 parts ofwater and 300 parts of 1,4-dioxane with stirring and heating. Themixture was mixed with 5 parts of activated carbon and filtered. Thesolution was allowed to cool slowly. The separated crystals of theadduct were filtered off and washed with 30% dioxane. It was thenrecrystallized from a mixture of 360 parts of water and 270 parts ofdioxane. The product was dried in vacuo over concentrated sulfuric acid.Melting point (after recrystallization from dilute ethanol): 247°-248°C.

    ______________________________________                                        Analysis (in % by weight):                                                                      C     H                                                     ______________________________________                                        calculated          67.24%  5.2%                                              found               67.15%  5.2%                                              ______________________________________                                    

    __________________________________________________________________________    Content of impurities in the starting product and the end product             (% by weight)                                                                             2-hydroxy-                                                                           2-hydroxy                                                              naphthalene-                                                                         naphthalene-                                                           3,6-dicar-                                                                           3-carboxylic 2,2-dihydroxy                                             boxylic acid                                                                         acid   2-naphthol                                                                          1,1'-dinaphthyl                                                                       Color value                           __________________________________________________________________________    Example                                                                            Starting                                                                             0.421  0.227  0.065 0.018   21.0 × 10.sup.-6                1    product                                                                       End product                                                                          0.018  <0.05  <0.05 <0.05    1.8 × 10.sup.-6                Example                                                                            Starting                                                                             1.762  0.719  0.225 <0.05   60.9 × 10.sup.-6                2    product                                                                       End product                                                                          0.085  <0.05  0.014 <0.05    1.8 × 10.sup.-6                __________________________________________________________________________

EXAMPLE 3

12.5 parts of crude 2,6-acid were dissolved in a mixture of 42 parts ofglacial acetic acid and 41 parts of dioxane with stirring and heating.The mixture was mixed with 1 part of activated carbon, stirred for 2minutes and then filtered. The crystalline adduct separated from thesolution on slow cooling; it was filtered off and washed with 20% byweight of ethanol. Drying in vacuo over concentrated sulfuric acidfurnished 11.3 parts of the adduct.

We claim:
 1. A dioxane adduct of 2-hydroxynaphthalene-6-carboxylic acidcontaining about 2 mol of 2-hydroxynaphthalene-6-carboxylic acid per molof 1,4-dioxane.